another secret to this amp would be that the filter caps must be of equal value, otherwise, the voltage developed across them will not be equal, thus contributing further to voltage offsets....i suppose you can install equal value resistors to force the issue...ecaps are notoriously wide tolerance caps, matching would then be desirable....this is perhaps the reason why another version used a center tapped power transformer....
Sir in models with R14,19 paired with a trimmer, the trimmer is actually AC coupled to the junction of R14 and R19 via a capacitor. And in DC-coupled models of QSC, totally wala na ang R14 and R19. *DC-coupled models have a transformer center tap connected to the junction of C3,4 and C13,14 much like a conventional power supply. However, unlike the conventional power supply, it is still not connected to ground.
Audio opamp nothwithstanding, perhaps one other reasons why some audio designers still shy away from opamps is the fact that opamps has a very high open loop gain. This necessitates the use of heavy negative feedback to get it to do some useful (linear) functions. Which is not bad at all at first glance. Negative feedback does a lot of wonderful things to an amplifier circuit, but it too has a bad side. Example of bad effects could be added instability. The good news is, instability can be easily tamed. Another bad effect is poorer transient response. In a negative feedback circuit, the fact that it takes a finite amount of time before the sampled output signal propagates back to the input means there will be a brief amount of time where the opamp “sees” no feedback at all. This phenomenon reveals itself as spikes in the output edges when the amplifier is fed with an input rectangular waveform. Minimizing this is the greater challenge.That's why some audio manufacturers uses as little negative feedback as possible, and advertises this as a desirable feature.
AC Coupling or DC Coupling?Labgruppen's categorizing the QSC amps models into AC coupled and DC coupled is quite interesting. I believe this is a good topic for discussion. Let us take a look at the 2 categories:It is interesting to note that the current paths when the upper half of both circuits conduct are the same. The only difference is the circuit without center tap connection will have a bigger tendency to have the voltage across C3/C4 sag because it has to replenish the charge being dumped through the speaker load via C13/C4. The circuit with center tap connection has a direct path for replenishing the charge being dumped through the speaker. It is also interesting to note that as far as the rest of the circuit is concerned, C3/C4 is just a very low impedance voltage source or a battery - irregardless of where the current used to charge it comes from.As long as the circuit operates normally, the operations are practically identical. Only when an output transistor gets shorted that a difference comes out. Under this condition, the circuit with center tap connection will continue to supply current to the short circuit section in the form of pulsating DC because C3/C4 can no longer charge up to the nominal +VS value.Now, let us compare the current paths of these 2 circuits to the current path in a conventional output stage of a LIN topology amp as shown below. Here, the load is generally considered as DC coupled. Do you see any difference in current path? Let's also compare them to the current path of what is generally accepted as an AC coupled amp.From this comparison, how do we categorize the QSC amp models?
Sir Lab what is your take about the operation of the amp?Is it the same with sir TTH?Do you have any other explanation how the circuit works? Just the output stage.
... The input signal is fed to its inverting input (U1B). Thus a positive going signal will make U1B's output go negative. This negative going signal makes Q3 conduct which in turn makes Q4 conduct. (When Q3 conducts, its collector pulls UP the base of Q4 and makes Q4 conduct) When Q4 conducts this will pull the negative rail up towards ground. Since the power supply is floating(not grounded), both the positive and negative rails will go up. This will make the output (center tap of C3,4 and C13/14) which was initially at ground level go higher that ground, thus a positive going output. The reverse is true for negative going signals...
Just a silly question sir TTH, if I use a sziklai pair as a switch, will it also be correct not to treat it as a sziklai pair? (Just to clarify, I understand your explanation and I am not contesting it. In fact when I read it, I thought "oo nga ano..") Now, I just had a musing that I can also apply the same explanation in sziklai pairs used as a switch. Example below, can I say that Q2 is like a VAS with a very high voltage gain (no Re + very high effective collector resistance), Q1 an EF for Q2 since Q1's emitter follows the voltage at the collector of Q2) thus not treating them as a sziklai pair? Now question again, when shall we treat a sziklai pair as sziklai pair? Because when we see the diagram of a sziklai pair, we can say that the right-side transistor will always be an emitter follower for the collector of the left-side transistor. Though this explanation can also explain why sziklai pairs have very high current gains. Since Q1 is acting like an EF for Q2, this means that Q2 sees only a very small fraction of the load current (ILOAD/BQ1). Thus Q2 requires only a very minimal base current to saturate ((ILOAD/BQ1) / BQ2). Thus the current gain of sziklai pair is BQ1*BQ2..EDIT:BQ1 = Beta Q1BQ2 = Beta Q2